And, by the way, your film must include an investigator character named John or Joyce. And a marshmallow. And the line, “I saw one of those yesterday.”

About 1,200 amateur filmmakers have been running around San Diego since Friday night on a whirlwind creative adventure called the 48 Hour Film Project.

It’s an international competition at least a decade old. What’s on the line are bragging rights and a shot at the Cannes Film Festival’s Short Film Corner.

This year, the 48 Hour Film Project will visit more than 120 cities — including far-flung spots such as Dubai and Nairobi — and include more than 60,000 filmmakers.

That’s why, on Saturday afternoon, patrons entering The Cardiff Office, a neighborhood bar just above the Coast Highway, found two camera guys, a sound man and several actors working in the wood-paneled barroom.

Director Samantha Goldstein called, “action,” and three of her actors plowed through dialogue written for them in the wee hours of the night before.

On Friday evening, San Diego teams received their scenario — the investigator, marshmallow, etc. — and went to work. Each team also got one individual requirement: genre.

Goldstein’s 16-person crew was told to make a road-trip movie.

Goldstein, who is a mom and a part-time writing teacher, has competed in the 48-hour event for the past 11 years.

“It’s the kind of thing where, at the end of it, you say, ‘Oh my god, it was the worst thing I’ve ever done. And when’s the next one?’” she said, laughing.

“There’s something about the adrenaline of it, and trying to beat the clock to do something creative.”

Meanwhile, in Mission Hills, the crew of another San Diego team spilled out onto a residential cul-de-sac.

Walkie-talkies crackled on their hips. People peered into cameras. Two young men deflected sunlight using large silver screens.

In the middle of it, Aly Bucholz Neely — her stomach round with a baby on the way — directed the chaos.

“That was money for me,” Bucholz Neely told her crew, as she stood with a hand-held camera. “What take was that?”

In real life, the 27-year-old is marketing director at Tallgrass Pictures, a San Diego film and photography company. That means she usually works way behind the scenes.

The weekend film contest is her one chance all year to be behind the camera. This is her sixth time competing.

“Every year, we just have a ball. It’s almost like, I wait half of the year just for 48,” Bucholz Neely said.

Fate handed them film noir as their genre. That presented a challenge because Bucholz Neely’s plan was to make her 8-year-old son, Kaiden, and a few other kids the main characters.

Still, they plowed forward, glad for the chance to solve a creative problem.

“We all have these left-minded jobs, our analytical jobs. And we get to do a right-minded activity,” Bucholz Neely said.

How will it all turn out? Films entered in the San Diego contest will be screened July 14 to 17 at the downtown Gaslamp 15 movie theater.